Critical randomly indexed branching processes

Bienaymé-Galton-Watson branching processes subordinated to a continuous time random index are considered. The branching processes are assumed to be critical with finite or infinite offspring variance. The indexing process is assumed to be a renewal one with finite or infinite mean of the interarrival times. Under these conditions we prove the asymptotic formulas for the first two moments and for the probability of non-extinction. We also obtain proper limiting distributions under suitable normalization.